In the construction of modern structures such as buildings and bridges, braced frames including beams, columns, and frame braces are arranged and fastened or joined together, using known engineering principles and practices to form a skeletal load resisting framework of the structure. The arrangement of the beams, also known as girders, columns, and braces and their connections are designed to ensure the framework can support the gravity and lateral loads contemplated for the intended use of the bridge, building or other structure. Making appropriate engineering assessments of loads and how these loads are resisted represents current design methodology. These assessments are compounded in complexity when considering loads for wind and seismic events, and determining the forces, stresses, and strains. It is well known that during an earthquake, the dynamic horizontal and vertical inertia loads and stresses and strains imposed on a structure have the greatest impact on the connections of the beams, columns, and braces which constitute the seismic damage resistant frame. Under high seismic or wind loading or even from repeated exposure to milder loadings, the connections in the structure may fail, possibly resulting in the collapse of the structure and the loss of life.
The beams and columns are typically, but not limited to, conventional rolled or built up steel I-beams, also known as W sections or wide flange sections, or box sections also known as tube sections. The frame brace members may have similar shapes as the beams and columns but may also be single or double angles or channels or tubular or tee shaped members. The beams, columns and braces are usually joined using what is known in the structural engineering profession as gusset plates. The presence of these gusset plates, which may be typically either bolted or welded to the joined members, causes the structure members to be rigidly joined so that the structural frame becomes, in essence, a braced-moment frame which results in unintentional overloading of the frame members (Richard 1986). Results of full scale tests conducted by Tsai et al. (2003), Lopez et al (2002, 2004), Gross (1990), and Roeder et al. (2004) demonstrate that stiff beam-column-brace connections attract large force and moment demands, which can lead to high moments and shears in the beams and columns. These unintentional high moments and shears in the joined members of the braced frame can result in premature fracture modes of the structural members when the frame is subjected to the design gravity, seismic, and wind loadings because these forces are not considered in the frame design. Evaluation of the full scale tests by Walters et al (2004) have shown that in conventionally designed braced frames, the moment frame action caused by the unintentional and undesirable beam and column moments and shears alone will provide a large part of the braced frame's resistance to lateral loads.
As previously stated, in conventionally braced frame designs, moment frame action caused by the gusset plates result in unintentional and undesirable moments and shears in the beams and columns. This can lead to fractures in the beam and column flanges and/or webs when the frame is subjected to lateral seismic or wind loading. Conventionally braced frame designs resist lateral load in a combination of braced frame action and moment frame action.
In the current practice of braced frame design, the beam-to-column connection at the brace gusset is normally a rigid welded and/or bolted assembly to the beam and column which creates a stiff moment resisting connection that generates moments and shears in the braced frame that are not accounted for in the braced frame design rationale. Both analytical studies and full scale tests have demonstrated the drift or displacement related joint rotation can result in the following potentially serious structural effects on the components of the braced frame: (1) a pinching or an in-plane crushing effect of the gusset plate which can lead to the buckling of the gusset plate; (2) overload of the welds and/or bolts of the gusset plate connections to the beam and column caused by the buckling of the gusset plate; (3) yielding and/or fracture of the beam and column flanges and/or webs due to high moments and shears in these components due to moment frame action that is not accounted for in conventional braced frame design rationale; and (4) unintended moment frame action that resists a large portion of the braced frame lateral loads rather than braces. This moment frame action is typically not accounted for in the design of the braced frame so that the force distribution in the braced frame is significantly different than the assumed design forces.